1. Field of the invention
The present invention relates to a lock-up clutch slip control for an automatic transmission that feedback-controls the engaging pressure of a lock-up clutch so that the slippage amount of the lock-up clutch will become equal to a target value.
2. Description of Related Art
A control apparatus for a vehicle hydrodynamic or hydraulic transmission having a direct-coupling clutch, such as a torque converter TC with a lock-up clutch LC, has been proposed which causes the direct-coupling clutch to slightly slip in order to, for example, absorb the periodical engine torque fluctuation that occurs during low speed traveling of the vehicle. The slippage amount herein means the difference between the rotational speed at the input of the direct-coupling clutch (usually, the engine speed) and the rotational speed at the output thereof (usually, the rotational speed of the transmission input shaft) (see FIG. 18A).
For example, a vehicle automatic transmission control apparatus described in Japanese examined patent application publication No. SHO 63-13060 (U.S. Pat. Nos. 4,466,311 and 4,468,988) controls the actual slippage amount of the direct-coupling clutch so as to become equal to a target slippage amount determined within a range of 10 to 20 rpm if it is determined that the traveling condition of the vehicle represented by the engine speed and the like is in a prestored slippage region.
However, in the conventional slip control apparatus for a vehicle hydrodynamic transmission having a direct-coupling clutch as described above, when the direct-coupling clutch is switched from a disengaged state to a slip-controlled state, the slippage amount is sharply reduced from a large amount allowed in the disengaged state. Such sharp reduction of the slippage amount causes the following drawbacks:
1. The sharp actual slippage reduction caused by the starting of the slip control as indicated by the solid line in FIG. 18B rapidly reduces the engine speed as indicated in FIG. 18C. As a result, the inertia of the engine is released to produce shocks, thus degrading the driving smoothness. PA1 2. At the start of the slip control, excessively large pressures are caused on the clutch because of the inertia of the hydraulic system. As a result, the direct-coupling clutch becomes temporarily but completely engaged (zero slippage) as indicated by the solid line in FIG. 18B, thus producing shocks.
This undesired event can be avoided by employing a slip control system that comprises a feedback compensation unit, a clutch pressure control unit and a lock-up clutch system and feeds back the actual slippage amount as illustrated in FIG. 19A to control the response of actual slippage amount (indicated by the solid line in FIG. 19B1) to the stepwise changes of the target slippage amount (indicated by the broken line in FIG. 19B) so as to achieve smooth transition free from undershooting as indicated by the dot-dash line in FIG. 18B. However, such a control system suffers other problems stated below in connection with the tuning of the control unit as described above.
In the case where the throttle opening (FIG. 19B2) is rapidly reduced so that the engine torque sharply decreases during the ordinary slip control by which the slippage amount is controlled to a predetermined value, the engaging pressure of the direct-coupling clutch needs to be reduced to maintain the slippage amount at the same level. However, since the control unit used in the aforementioned control system has been tuned so as to have moderate responsiveness, the correction of the engaging pressure is delayed, and the direct-coupling clutch becomes completely engaged for a short time, thus producing shocks.